Coherent optical receivers process traffic, in the form of optical signals, received from an optical network. Coherent optical receivers process the optical signals by performing operations on the optical signals, such as polarization beam splitting, demodulation, analog-to-digital conversion, equalization, etc.
The coherent optical receivers may use different modulation techniques (e.g., phase shift keying, frequency modulation, etc.) to process traffic received from the optical network. Examples of common phase shift keying techniques include polarization multiplexed (PM) binary phase shift keying (PM-BPSK), and PM-quadrature phase shift keying (PM-QPSK) (sometimes referred to as “four-quadrature amplitude modulation (4QAM)”). PM-BPSK uses two bits to represent a dual-polarized symbol and PM-QPSK uses four bits to represent a dual-polarized symbol. Recently, another phase shift keying technique has emerged that represents a hybrid of the PM-BPSK and PM-QPSK techniques (sometimes referred to as “three-quadrature amplitude modulation (3QAM)” or “HEXA”). 3QAM uses three bits to represent a dual-polarized symbol. Unfortunately, coherent optical receivers, that process PM-QPSK and/or PM-BPSK traffic cannot process 3QAM traffic without incurring false locks that cause catastrophic bit error rates and/or a disruption of service.